Nonaqueous electrolyte batteries (for example, lithium ion batteries) having a high energy density attract people's attention as a large-sized and large-capacity power supply used for electric vehicles (EV), hybrid electric vehicles (HEV), motor-driven bikes, forklifts and the like. Development to make the lithium ion battery larger in size and to increase the capacity is under way while considering long life and safety. As a large-capacity power supply, battery packs in which a large number of batteries are connected in series or in parallel to increase driving power have been developed.
As a nonaqueous electrolyte battery, particularly as a positive electrode current collector, an aluminum substrate is frequently used due to its oxidation resistance.
In recent years, batteries using lithium titanate for the negative electrode have been developed. Some batteries using lithium titanate for the negative electrode use an aluminum substrate for the negative electrode.
On the other hand, plural current collector tabs for electric conduction are more often pulled out from an electrode to achieve higher output. Ultrasonic bonding is suitable to bond plural current collector tabs together, and the ultrasonic bonding is also suitable to bond plural current collector tabs to an external lead or the like connected to a battery case. For example, it is difficult for a laser to bond plural metal plates into one member. Moreover, if current collector tabs containing an aluminum material are welded by resistance, though aluminum itself has a low melting point and a low boiling point, the aluminum material is likely to burst apart at the moment of bonding because an oxide film formed on the surface of the aluminum material has a high melting point.
When plural current collector tabs are bonded together, yields are adversely affected in most cases if positions of the current collector tabs are not aligned due to a problem of machine accuracy or the like regardless of the method of bonding. If tabs are badly aligned, for example, one of a positive electrode current collector tab and a negative electrode current collector tab may come into contact with an insulator that separates both current collector tabs to induce an internal short-circuit or current collector tabs may inhibit other components or other processes. When plural current collector tabs are bonded to a lead by ultrasonic bonding, and then the lead is bonded to another lead by laser welding, if the tabs are not well aligned, a protruding tab may overlap with a portion to be welded by laser, which may cause poor laser welding and result in lower yields.